Add postprocessor for number of OpenMC cells

[aprilnovak]

This is a useful feature for the on-the-fly skinning, to see how many cells have been formed.

[moosebuild]

Job Precheck on 2f556da wanted to post the following:

Your code requires style changes.

A patch was auto generated and copied here
You can directly apply the patch by running, in the top level of your repository:

curl -s https://mooseframework.inl.gov/cardinal/docs/PRs/857/clang_format/style.patch | git apply -v

Alternatively, with your repository up to date and in the top level of your repository:

git clang-format cb6120bbc7b73259b99fbc53de423ea284f5bc4f

[moosebuild]

Job Documentation on 7343c14 wanted to post the following:

View the site here

This comment will be updated on new commits.

[moosebuild]

Job Coverage on 7343c14 wanted to post the following:

Coverage

	cb6120	#857 7343c1
	Total	Total	+/-	New
Rate	93.53%	93.53%	-0.00%	90.00%
Hits	7248	7257	+9	9
Misses	501	502	+1	1

Diff coverage report

Full coverage report

This comment will be updated on new commits.